Sebastian Beil

A photo‐responsive system where structure formation is coupled to catalytic activity is presented. The observed catalytic activity is reliant on intermolecular cooperative effects that are present when amphiphiles assemble into vesicular structures. Photo‐responsive units within the amphiphilic pre‐catalysts allow for switching between assembled and disassembled states, thereby modulating the catalytic activity. The ability to reversibly form cooperative catalysts within a dynamic self‐assembled system represents a conceptually new tool for the design of complex artificial systems in water.

A hydrogel resembling a Rubik's Cube, a trademark of Rubik's Brand Limited, is made via controllable dynamic covalent interactions. Its layers can be rotated either horizontally or vertically to produce new patterns. Ex situ modification or a chemical stimulus can also produce new color arrangements. The creation of multiple patterns may allow for potential applications in patterns‐related material research.

Abstract

The dynamic behavior of a macroscopic adhered hydrogel stabilized through controllable dynamic covalent interactions is reported. These interactions, involving the cross‐linked formation of a hydrogel through reaction of a diacylhydrazine precursor with a tetraformyl partner, increase as a function of time. By using a contact time of 24 h and different compounds with recognized aggregation‐induced emission features (AIEgens), it proves possible to create six laminated acylhydrazone hydrogels displaying different fluorescent colors. Blocks of these hydrogels are then adhered into a structure resembling a Rubik's Cube, a trademark of Rubik's Brand Limited, (RC) and allowed to anneal for 1 h. This produces a 3 × 3 × 3 block (RC) wherein the individual fluorescent gel blocks are loosely adhered to one another. As a consequence, the 1 × 3 × 3 layers making up the RC can be rotated either horizontally or vertically to produce new patterns. Ex situ modification of the RC or application of a chemical stimulus can be used to produce new color arrangements. The present RC structure highlights how the temporal features, strong versus weak adhesion, may be exploited to create smart macroscopic structures.

Light‐induced phenomena abound in nature and have served as inspiration in the development of light‐driven materials for device applications. In this work, visible‐light‐induced formation of a helical superstructure is accomplished in orientationally ordered fluids, i.e., nematic liquid crystals, enabled by a visible‐light‐driven chiral molecular switch.

Abstract

Light‐induced phenomena occurring in nature and in synthetic materials are fascinating and have been exploited for technological applications. Here visible‐light‐induced formation of a helical superstructure is reported, i.e., a cholesteric liquid crystal phase, in orientationally ordered fluids, i.e., nematic liquid crystals, enabled by a visible‐light‐driven chiral molecular switch. The cyclic‐azobenzene‐based chiral molecular switch exhibits reversible photoisomerization in response to visible light of different wavelengths due to the band separation of n–π* transitions of its trans‐ and cis‐isomers. Green light (530 nm) drives the trans‐to‐cis photoisomerization whereas the cis‐to‐trans isomerization process of the chiral molecular switch can be caused by blue light (440 nm). It is observed that the helical twisting power of this chiral molecular switch increases upon irradiation with green light, which enables reversible induction of helical superstructure in nematic liquid crystals containing a very small quantity of the molecular switch. The occurrence of the light‐induced helical superstructure enables the formation of diffraction gratings in cholesteric films.

The selectivity and functional variability of porphyrin cofactors are typically based on substrate binding of metalloporphyrins wherein the pyrrole nitrogen units only serve to chelate the metal ions. Yet, using the free base porphyrin inner core system for other functions is possible through conformational engineering. As a first step towards porphyrin ‘enzyme‐like’ active centers, a structural and spectroscopic study of substrate binding to the inner core porphyrin system shows that a highly saddle distorted porphyrin with peripheral amino receptor groups [2,3,7,8,12,13,17,18‐octaethyl‐5,10,15,20‐tetrakis(2‐aminophenyl)porphyrin] coordinates analytes in a switchable manner dependent on the acidity of the solution. The supramolecular ensemble exhibits exceptionally high affinity to the pyrophosphate anion (2.26 ± 0.021)⋅109 M‐1 and does so in preference to other anions. 1H NMR spectroscopic studies provided insight into the likely mode of binding action and the characterization of atropisomers, all four of which were also studied by X‐ray crystallography.

A Ni‐catalyzed halogenation of alkenyl triflates enables the synthesis of a broad range of alkenyl iodides, bromides, and chlorides under mild reaction conditions. The reaction utilizes inexpensive, bench‐stable Ni(OAc)₂⋅4 H₂O as a precatalyst in the presence of a sub‐stoichiometric amount of Zn and either 1,5‐cyclooctadiene (cod) or 4‐(N,N‐dimethylamino)pyridine (DMAP).

Abstract

A Ni‐catalyzed halogenation of enol triflates was developed and it enables the synthesis of a broad range of alkenyl iodides, bromides, and chlorides under mild reaction conditions. The reaction utilizes inexpensive, bench‐stable Ni(OAc)₂⋅4 H₂O as a precatalyst and proceeds at room temperature in the presence of sub‐stoichiometric Zn and either 1,5‐cyclooctadiene or 4‐(N,N‐dimethylamino)pyridine.

Good as gold: The high oxidation potential between Au^I and Au^III means that gold redox catalysis typically requires stoichiometric amounts of a strong oxidant. An electrochemical approach is reported for the gold‐catalyzed oxidative coupling of terminal alkynes in which the oxidation of Au^I to Au^III is achieved through anode oxidation. This approach provides facile access to symmetrical or unsymmetrical conjugated diynes through homo‐coupling or cross‐coupling in up to 95 % yields.

Abstract

Due to the high oxidation potential between Au^I and Au^III, gold redox catalysis requires at least stoichiometric amounts of a strong oxidant. We herein report the first example of an electrochemical approach in promoting gold‐catalyzed oxidative coupling of terminal alkynes. Oxidation of Au^I to Au^III was successfully achieved through anode oxidation, which enabled facile access to either symmetrical or unsymmetrical conjugated diynes through homo‐coupling or cross‐coupling. This report extends the reaction scope of this transformation to substrates that are not compatible with strong chemical oxidants and potentiates the versatility of gold redox chemistry through the utilization of electrochemical oxidative conditions.

Nature Chemistry, Published online: 12 August 2019; doi:10.1038/s41557-019-0297-7

Although they are synthetically tunable, organic molecules that undergo singlet fission (the generation of two excitons from one photon) have not demonstrated the excited-state properties necessary to improve optoelectronic devices. Now, a general ‘energy cleft’ molecular design scheme has been demonstrated that enables rapid generation and long lifetimes of multiple triplet excitons that are for device applications.

Nature Communications, Published online: 08 August 2019; doi:10.1038/s41467-019-11467-4

Little is known about interactions between two antiaromatic molecules. Here, the authors synthesised a cyclophane, in which two antiaromatic porphyrin moieties adopt a stacked face-to-face geometry with a distance shorter than the sum of the van der Waals radii of the atoms involved.

Negative isn't always bad: Reduction of a porphyrin nanoring by the addition of 4 or 6 electrons was found to result in global (anti)aromaticity (see picture), as revealed by ¹H NMR spectroscopy and DFT calculations. Thus, the Hückel rules apply to huge macrocyclic anions, and the charge in these anions is fully delocalized.

Abstract

Doping, through oxidation or reduction, is often used to modify the properties of π‐conjugated oligomers. In most cases, the resulting charge distribution is difficult to determine. If the oligomer is cyclic and doping establishes global aromaticity or antiaromaticity, then it is certain that the charge is fully delocalized over the entire perimeter of the ring. Herein we show that reduction of a six‐porphyrin nanoring using decamethylcobaltocene results in global aromaticity (in the 6− state; [90 π]) and antiaromaticity (in the 4− state; [88 π]), consistent with the Hückel rules. Aromaticity is assigned by NMR spectroscopy and density‐functional theory calculations.

Splitting the work: Energy transfer (EnT) and photoinduced single‐electron transfer (SET) allow the reduction of arenes. Subsequent hydrogen‐atom transfer (HAT) yields the reduced dearomatized products.

Abstract

The direct reduction of arenes and heteroarenes by visible‐light irradiation remains challenging, as the energy of a single photon is not sufficient for breaking aromatic stabilization. Shown herein is that the energy accumulation of two visible‐light photons allows the dearomatization of arenes and heteroarenes. Mechanistic investigations confirm that the combination of energy‐transfer and electron‐transfer processes generates an arene radical anion, which is subsequently trapped by hydrogen‐atom transfer and finally protonated to form the dearomatized product. The photoreduction converts planar aromatic feedstock compounds into molecular skeletons that are of use in organic synthesis.

Nature Chemistry, Published online: 05 August 2019; doi:10.1038/s41557-019-0295-9

Studies into gold-catalysed cross-coupling reactions have expanded over recent decades; however, oxidative addition to gold(i) complexes remains challenging. Now it has been shown that a dual catalytic transformation involving iridium photosensitization to trigger oxidative addition to organogold intermediates enables C(sp2)–C(sp) cross-coupling reactions that are useful for the alkynylation of benzofurans.

Under strain: 1,3‐dithiolium‐4‐olates reacted with strained alkynes, affording polyaromatic thiophene structures in high yields. Their use for ligation applications has also been investigated with cyclooctynes, offering a new orthogonal reaction to the strain‐promoted azide–alkyne reaction.

Abstract

Reported here is the reactivity of mesoionic 1,3‐dithiolium‐4‐olates towards strained alkynes, leading to thiophene cycloaddition products. In the process, the potential of these dipoles towards orthogonal reaction with azides, allowing efficient double ligation reactions, was discovered. A versatile process to access benzo[c]thiophenes, in an unprecedented divergent fashion, was developed and provides a new entry to unconventional polyaromatic thiophenes.

The scalable synthesis of carboxy‐MIDA‐boronate, one of the simplest boron‐containing building blocks, is described. Carboxy‐MIDA‐boronate can undergo a diverse range of transformations, providing access to an extensive scope of borylated products. Carboxy‐MIDA‐boronate is found to possess unusual reactivity towards nucleophiles.

Abstract

The synthesis and applications of carboxy‐MIDA‐boronate, a novel C1 building block, are described. This molecule is accessible via a ruthenium tetraoxide‐mediated cleavage of commercially available ethynyl‐MIDA‐boronate. In the course of this study, carboxy‐MIDA‐boronate was found to possess ambident reactivity towards nucleophiles. Carboxylic acid derivatization produces a broad range of previously unknown carbamoyl‐, oxycarbo‐ and thiocarboboronates. Carboxy‐MIDA‐boronate and its derivatives undergo condensations to access borylated heterocycles with boron at positions that are difficult to access using alternate methods. The resulting heterocycles participate in the Suzuki–Miyaura cross‐coupling reaction, enabling entry into diverse bis(heteroaryl) motifs. The carbon monoxide‐releasing capacity of carboxy‐MIDA‐boronate was also examined and applied in palladium‐catalyzed carbonylation.